Appraisal of Some One-Center Calculations for the Neon- and Argonlike Series of Molecules

Abstract

The united-atom (UA) method and the self-consistent-field one-center-expansion (OCE) treatment have been applied recently to the study of neonlike and argonlike molecules, namely, HF, H2O, NH3, NH4+, CH4, HCl, H2S, PH3, PH4+, and SiH4. In the present article, the values of physical properties predicted by each method are compared with the corresponding experimental quantities. For each molecule, comparisons with experiment have been made for the molecular energy, theoretical bond lengths, ``breathing'' force constants, and, for NH3 and PH3, the proton affinities. The radial density distribution of electrons, determined by us from the published OCE wavefunctions, has been used to calculate the coherent x-ray scattering factor for the gaseous state of each molecule. The electron densities were also used to determine the purely diamagnetic contribution χr to the total molar magnetic susceptibility. A comparison was made with the corresponding UA results and with the available experimental quantities. From the trend shown by the relationship between the theoretical and ``experimental'' values for χr, it was possible, for a molecule with Td symmetry, to suggest a range of values for the diagonal components (gjj″) of the electronic contribution (g″) to the molecular rotational gyromagnetic tensor (g). Hence, for SiH4, it was suggested that −1.48 < gjj″ < −1.31. No experimental value for this quantity is known by the authors. Although it is seen that the OCE results are, in most instances, superior to the results obtained from the UA approximation, the latter method still provides very useful results with comparatively little computation.